Conventionally, metal materials including a particular aluminum alloy, a titanium alloy or a stainless steel have been mainly used as a structural member of an aircraft such as a wing, body, a window frame or a circular bore portion such as a fuel inlet, because of having sufficient mechanical strength to be used in the cited sections, and strength characteristics appropriate to stress characteristics of these sections. However, in order to satisfy a strength requirement of an aircraft, a multitude of metal structural members, each of which has a considerable weight, have to be used; therefore a total weight of the aircraft inevitably becomes heavy. Accordingly, degree of designing freedom is significantly restrained, and in a practical aspect a huge amount of fuel is consumed in each flight; therefore making light-weighted aircraft components has been a critical issue from the viewpoint of manufacturing, operation and safety of an aircraft.
Lately studies have been progressing on manufacturing a structural member of an aircraft with a fiber-reinforced composite material having a high specific strength constituted of a reinforced fiber including a glass fiber, an aramid fiber or a carbon fiber impregnated with a polymer matrix such as an epoxy resin, and currently a considerable number of materials are in practical use. It is known in the art that a composite material has a superior specific strength and specific rigidity to metal materials, and it is also well known that a composite material offers higher degree of freedom in the structure design because of anisotropy that is characteristic of a reinforced fiber. Besides, since formation of a single piece of large-scale structural member is possible, a number of parts can be substantially reduced, and resultantly the manufacturing cost can be lowered.